By monitoring the transformation process using X-rays, they were able to identify the places and their properties that were the most likely starting points for the nucleation process of the transition from ferrite to austenite.

Evolution of the microstructure at the level of grains as measured during the α → γ phase transformation. Microstructure (a) before the phase transformation at 1103 K, (b,d) 1109 K (c,e), 1113 K, (f,h) 1117 K, (g,i) 1119 K, and (j) after the phase transformation at 1121 K. Position of (a–c,f,g) the α grains and (d,e,h–j) the γ grains. The axis units are micrometers. Grains are represented as spheres. The size of each marker is equal to the size of the corresponding grain divided by 2.5, which is needed to visualize the microstructure. The two blue circles in each plot indicate the size of the sample cylinder (500 μm diameter). Colors represent orientation according to the colour legend (Euler angles). Sharma et al. Click to enlarge.

Alloys and microstructures. Up until now, the most widely used method for changing the properties of steel is the addition of elements affecting the structure, and therefore the properties, of the resulting steel type—i.e., alloys. Alloys can consist of more than ten different types of elements, making both manufacturing them and recycling them complicated and often expensive.

Insight into the nucleation process may yield a solution. If one could direct this process, one would theoretically be able to create microstructures that give the material the desired macroscopic properties, without the addition of other (rare) elements.

Steel must be both strong and sufficiently deformable in order to, for example, make components lighter in the automotive industry. When manufacturing so-called electrical steel for transformer cores and fire-resistant steels for high-rise buildings, you are also looking for specific properties. The nucleation process is the key to limiting the use of alloying elements in these processes. We have now found places where an iron crystal tends to nucleate.

If you can make metals less complex—i.e. use less alloying elements and do more with the microstructure—you can also keep the bulk metals simpler. That would make production less expensive, and it would also make it easier to reuse the metals when recycling a mixture of different steel grades, because they will have the same or similar composition. You would therefore have to experiment more with the structure than with the alloying. The next step is to replace critical alloying elements in metals and to reduce the number of different alloying elements and the concentration of the alloying elements by optimizing the microstructure.

—TU Delft materials scientist Erik Offerman

Offerman formulated a hypothesis on the activation energy of the nucleation process fourteen years ago, which was harshly criticized by one of the experts in the field at that time. With this article, he was finally able to prove his hypothesis.

This research was partly made possible by the Netherlands technology foundation STW.

Resources

• H. Sharma, J. Sietsma & S. E. Offerman (2016) “Preferential Nucleation during Polymorphic Transformations” Scientific Reports doi: 10.1038/srep30860